Objective: Converging evidence indicates large magnitude deficits in the "working" component of working memory for children with ADHD. However, our understanding of the relation between these central executive deficits and ADHD behavioral symptoms remains limited due to problems with several commonly used working memory tests. Method: Children with ADHD (n = 25) completed a counterbalanced series of working memory tasks that differed only in memory set predictability. Results: Results indicated that central executive demands increased when memory set was unpredictable, as evidenced by moderate performance decreases (d = 0.22-0.56) and large changes in performance variability (d = 0.93-3.16) and response times (d = 1.74-4.16). Activity level remained relatively stable when memory set was unpredictable but decreased significantly over time when memory set was predictable. Conclusion: Results suggest that altering memory set predictability is a feasible method for increasing/maintaining central executive demands over time, and suggest a positive association between working memory demands and gross motor activity for children with ADHD.